Recently, there has been developed a sensing device for quantifying the concentration of analytes based on the intensity of fluorescence, which changes due to an interaction between the analytes and a labeled compound. One proposed application is a device for continuously quantifying the concentration of glucose with a sensor embedded in the body of an examinee (see U.S. Pat. No. 6,330,464). By using the device to acquire and analyze time-series data of the blood sugar levels of a diabetic patient, it is possible to appropriately establish a drug administration protocol for stabilizing the blood sugar level, and to offer guidance to the patient on how to improve his or her lifestyle habits.
A fluorescence signal that corresponds to a glucose concentration is obtained from a fluorochrome compound, which combines with glucose, for example, to emit fluorescence (see Japanese Patent No. 2883824). As disclosed in Japanese Patent No. 3296556, it has also been proposed to use, as a fluorescence signal, a change in the extent of a fluorescence resonance energy transfer, which occurs when a compound of fluorescein-labeled dextran and rhodamine-labeled concanavalin A, which does not cause a fluorescence signal change simply by combining with glucose, is dissociated by glucose, and to correlate the fluorescence signal with glucose concentration.
In the absence of an analyte concentration change, i.e., in a state of equilibrium, an analyte concentration [A(t)] and a fluorescence intensity F(t) are related to each other according to the following equation (1):−(α1[A(t)]+α2)F(t)+α3[A(t)]=0  (1)where α1 through α3 represent quantification coefficients attributed to a reaction rate constant, etc. In particular, α2 corresponds to a reaction rate constant in relation to dissociation between the labeled compound and a third compound. Equation (1) may be simplified in order to calculate [A(t)] according to the following equation (2):
                              [                      A            ⁡                          (              t              )                                ]                =                                            α              2                        ⁢                          F              ⁡                              (                t                )                                                                        α              3                        -                                          α                1                            ⁢                              F                ⁡                                  (                  t                  )                                                                                        (        2        )            
According to such a system, the analyte concentration [A(t)] may be quantified continuously by acquiring the fluorescence intensity F(t) at a predetermined quantification time t, and quantifying the acquired fluorescence intensity F(t) according to equation (2).